The present invention relates to photopolymerizable flexographic printing elements which contain cyclohexanepolycarboxylic esters as plasticizers and also their use for producing flexographic printing forms for printing with UV inks, in particular for UV narrow web printing.
Flexographic printing by means of UV inks is known in principle. The technique is used, in particular, for producing labels. In UV narrow web printing, a web, for example of paper or self-adhesive plastic films, is printed with the label pattern. The printing machines used for this purpose have printing cylinders having a relatively small diameter, for example a diameter of only from 6 to 12 cm. When mounting on the printing cylinders, the flexographic printing plates used accordingly have to be very flexible so that they can readily be bent around the small printing cylinder.
UV flexographic printing inks contain, for curing, monomers having ethylenically unsaturated groups, for example hexanediol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate or else ethoxylated monomers such as ethoxylated tripropylene glycol diacrylate or ethoxylated trimethylolpropane triacrylate. Such monomers can swell flexographic printing plates over the course of time. However, swelling of the flexographic printing plate leads with increasing duration of the printing operation to undesirable changes in the printed image because fine print elements experience a volume increase due to the swelling and at the same time the hardness of the flexographic printing plate can decrease. The individual elements of the plate become broader, the shade value increase goes up and fine picture elements can run together.
The swellability of the flexographic printing plate is naturally influenced by the degree of crosslinking. The higher the degree of crosslinking, the lower the swellability of the plate. Relatively high crosslinking also increases the hardness of the flexographic printing plates under otherwise the same conditions, which can be thoroughly desirable in halftone printing. At the same time, however, the flexibility of the flexographic printing plate also decreases, i.e. it is more difficult to bend so that mounting on the printing cylinder having a small diameter becomes difficult.
It has been proposed that specific binders be used in the flexographic printing plates for printing with UV inks. EP 833 206 A and EP 833 207 A propose using specific block copolymers having styrene blocks and also isoprene or butadiene-isoprene blocks, with the peak temperature of the primary dispersion of tan δ being not more than 30° C. EP 992 849 A1 proposes using block copolymers having at least one styrene block and at least one styrene-butadiene block.
The addition of plasticizers to flexographic printing plates is known. On this subject, reference may be made by way of example to EP 992 849 A1, paragraph [0021]. White oils or oligomeric plasticizers, in particular polybutadiene oils, are particularly widespread as plasticizers in flexographic printing plates.
The flexibility of flexographic printing plates can in principle be increased by the use of plasticizers. However, relatively large amounts of plasticizers at the same time also decrease the degree of crosslinking, so that the flexographic printing plate becomes softer again and accordingly also swell to a greater degree again on contact with UV inks.
It was an object of the invention to improve the flexibility of flexographic printing plates for printing with UV inks, in particular UV narrow web printing.